Change an Object's Material to another Material with Python [Pic Attached]
bpy.data.objects['Paper_obj'].data.materials[3] = bpy.data.materials['PaperTopRed']


The above should be the general answer.

However, what if I do not want to use the slot number but would like to target "PaperTopYellow" and replace it with "PaperTopRed"?

Sometimes, the slot number for "PaperTopYellow" may not be 3 but 2 instead.